integration-singleton.md

title	Singletons for Stateless Services
impact	MEDIUM
impactDescription	Avoid redundant service instantiation
tags	integration, singleton, stateless, performance

Singletons for Stateless Services

Impact: MEDIUM (avoid redundant service instantiation)

When testing with a stateless service that has expensive construction (e.g., loading configuration, compiling schemas), use a singleton pattern in your test to avoid re-creating it for every test method.

This is appropriate only for truly stateless, immutable services. If the service holds mutable state, use setUp() instead.

Incorrect (re-creating stateless service per test):

<?php

declare(strict_types=1);

namespace App\Tests;

use App\SchemaValidator;
use PHPUnit\Framework\Attributes\Test;
use PHPUnit\Framework\TestCase;

final class SchemaValidatorTest extends TestCase
{
    #[Test]
    public function it_validates_correct_schema(): void
    {
        // Expensive: loads and compiles JSON schema on every test
        $validator = new SchemaValidator(__DIR__ . '/fixtures/schema.json');

        $this->assertTrue($validator->validate(['name' => 'John']));
    }

    #[Test]
    public function it_rejects_invalid_schema(): void
    {
        $validator = new SchemaValidator(__DIR__ . '/fixtures/schema.json');

        $this->assertFalse($validator->validate(['invalid' => true]));
    }
}

Correct (singleton for stateless service):

<?php

declare(strict_types=1);

namespace App\Tests;

use App\SchemaValidator;
use PHPUnit\Framework\Attributes\Test;
use PHPUnit\Framework\TestCase;

final class SchemaValidatorTest extends TestCase
{
    private static SchemaValidator $validator;

    public static function setUpBeforeClass(): void
    {
        self::$validator = new SchemaValidator(__DIR__ . '/fixtures/schema.json');
    }

    #[Test]
    public function it_validates_correct_schema(): void
    {
        $this->assertTrue(self::$validator->validate(['name' => 'John']));
    }

    #[Test]
    public function it_rejects_invalid_schema(): void
    {
        $this->assertFalse(self::$validator->validate(['invalid' => true]));
    }
}